Onan Generator – low oil pressure fault (bad sensor?)

[Kraig Kilger]

I have an Onan MDKBM 11.5 KW.  It is well maintained with approx. 1800 total hours and has been without issue until this weekend when it shut down twice, each time after running for a couple hours.  On each occasion the unit provided a diagnostic “two blink” light which indicates low oil pressure.  The oil level is at the top line on the dip stick, and the oil and filter were changed 8 months ago with approx. 50 hours on the unit since.   My initial reaction is that the oil pressure sensor may be going bad and I should replace it as a starting point but I wanted to see if others have encountered similar issues and if so whether I am on the right track.  Thank you.        

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[Kraig Kilger]

Thank you.  Sourcing a new sensor was easy and inexpensive so I swapped it out.  Ran it a couple of times for about 30 minutes each with the new sensor and seems fine but will be on the boat for 3 days/nights this weekend so will find out for sure whether the sensor was the issue.  Given that when it shut down with the low oil pressure fault it had been running for well over an hour to me suggests that oil pressure is fine.  Of course, if the issue persists then a more in depth troubleshooting effort is warranted.  At that point, I will change the oil and filter, and test the old oil for fuel dilution, etc…  One step at a time.  Fingers crossed that the simplest answer is the correct one! 

[Clark Leighs]

I am not a generator mechanic or even close but:

First I would jumper, bypass,  the pressure sensor.     

I would also  install a mechanical pressure guage in place of the sensor to find out if the  oil pressure is actually good or bad.  

Add the guage on a TEE so both are in place.     Start the engine.  If the mechanical guage shows good pressure then it is the sensor.    The sensor is actually a switch set to close at a minimum oil pressure which allows the engine to continue running.     If that minimum is not attained then when you release the start button the engine will quit.

Be prepare to shut the engine down quickly if the pressure sensor is correct.      You may want  a helper if you have a long ways to go to shut it down.   If you have an on the gen control   with shutdown then you could DIY.

 

I think in your case I would at least investigate the cost of a new sensor and maybe just replace it. I am not usually one to just throw parts at a problem but sometimes if the parts are not expensive and the problem is intermittent so you cannot watch it  but it is causing trouble then sometimes this would be the best way to go since the indicator light show the sensor tripped.

JM O

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